Minimum fluidization superficial velocity mixing

In equation 2.28 Up is the lower of the two minimum fluidizing velocities of the two types of particle in the mixture and Ufo is the velocity at which mixing takes over or begins to dominate segregation. Thus, as the superficial gas velocity in the bed is increased, the mixing index increases from M = 0 at the lower minimum fluidizing velocity (m = Mp), where the bed is quiescent with no particle movement because of the absence of bubbles, to M = 0.5 when, by definition, the velocity is equal to Uto- The mixing index approaches a value of unity as the velocity increases still further (Nienow and Chiba, 1985). 

Because it is the bubbles which are largely responsible for mixing in fluidized beds, it is important to know how much of the gas flowing into the bed forms bubbles. A first estimate of the bubble flow rate, (2b> is provided by the two phase theory. This pictures the bed as consisting of a dense phase in which the gas velocity is equal to that at minimum fluidization, U f, and a bubble phase which carries all the additional gas. Thus, at a superficial velocity U ... 

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